This invention relates to the field of photonic crystal fibers.
A photonic crystal fiber is a special form of optical fiber. Single-mode optical fibers are widely used in applications such as telecommunications and sensing. Such fibers are typically made entirely from solid transparent materials such as glass and each fiber typically has the same cross-sectional structure along its length. The transparent material in one part (usually the middle) of the cross-section has a higher refractive index than the rest and forms an optical core within which light is guided by total internal reflection. We refer to such a fiber as a standard fiber.
There are many well-established techniques and machines for handling and processing standard fibers; for example, cleavers use a hard knife-edge to break a fiber, giving a clean end-face, and fusion splicers use a hot electric arc to join two fibers together at their ends. One process, fusion-tapering, is used to make a variety of fiber devices for performing some function on light passing along a fiber. In that process, a fiber is locally heated until it softens and then it is stretched so as to narrow the fiber locally in the heated region. Light passing along the fiber is affected by the narrowness of the treated region. In a typical tapered single-mode fiber, the light spreads out from the core and occupies more of the surrounding cladding. If the fiber is sufficiently narrowed, the light spreads out from the core completely and is then guided by the outer boundary of the entire fiber. The fiber is typically heated by immersion in a gas flame, proximity to an electrical heater or exposure to an intense laser beam.
A tapered fiber that is cleaved at the narrowest point of the taper can act as a beam expander because the light wave has a greater cross-section at the cleave than it has in the untreated fiber. Such a beam expander can assist the launching of light into the fiber and extraction of light from the fiber.
A fiber that is tapered so that light is locally guided at its outer boundary can act as a local optical sensor. In the tapered region, the light is sensitive to the medium surrounding the fiber, whereas elsewhere it is insensitive because it is buried in the central core.
Two or more fibers that are tapered together in parallel contact can act as a fiber beam-splitter (or directional coupler), in which at least some of the light in one fiber transfers across to the other fiber in the narrowed region.
In the last few years a non-standard type of optical fiber has been demonstrated, called the photonic-crystal fiber (PCF). Typically, this is made from a single solid, and substantially transparent, material, such as fused silica glass, within which is embedded a periodic array of air holes, running parallel to the fiber axis and extending the full length of the fiber. A defect in the form of a single missing air hole within the regular array forms a region of raised refractive index, which acts as a waveguiding fiber core within which light is guided, in a manner analogous to total-internal-reflection guiding in standard fibers. Another mechanism for guiding light is based on photonic-band-gap effects rather than total internal reflection. Photonic-band-gap guidance can be obtained by suitable design of the array of air holes. Light of some propagation constants can be confined to the core and will propagate therein.
Photonic-crystal fiber can be fabricated by stacking glass capillaries and canes on a macroscopic scale into the required shape, and then holding them in place while fusing them together and drawing them down into a fiber.
The PCF has a number of technologically significant properties; for example, it can support a single-mode over a very broad range of wavelengths, it can have a large mode area and thus carry high optical powers, and it can have a large normal dispersion at the telecommunications wavelength of 1.55 microns. PCFs are typically not circularly symmetric, as a result of the stack-and-draw method typically used to make them.
Technological application of PCFs would be facilitated by handling and processing techniques parallel to those described above for standard fibers. Unfortunately, some of those techniques are not appropriate for PCFs; for example, an attempt to fusion splice two PCFs together causes the air inside them to expand explosively, destroying the fiber ends being joined.
It is an object of the invention to provide PCF optical devices analogous to standard fiber devices. It is another object of the invention to provide a process for producing such devices.
According to the invention there is provided a photonic crystal fiber including a plurality of longitudinal holes, in which at least some of the holes have a different cross-sectional area in a first region of the fiber, that region having been heat-treated after fabrication of the fiber, from their cross-sectional area in a second region of the fiber, wherein the optical properties of the fiber in the heat-treated region are altered by virtue of the change in cross-sectional area of the holes in that region.
The words xe2x80x9cafter fabricationxe2x80x9d should be taken to mean any time after the fiber has been drawn.
The heat treatment processes that can be used are typically the same as those described above for fusion tapering of standard fibers. As with standard fibers, the heat treatment can be accompanied by stretching, to narrow the fiber down. In contrast to standard fibers, however, changes in optical properties can result without stretching the fiber at all; that is because heat treatment can allow some or all of the holes in the fiber to collapse partially or completely under the influence of surface tension. That can be achieved either with or without simultaneous stretching. Furthermore, if some of the holes are pressurized, they can be made to expand instead of collapse, and differential pressurization of the holes can, in principle, be used to create any pattern of hole collapse and expansion. As with the tapering of standard fibers, for most applications, the transitions between untreated fiber and the middle of the heat treated region must be sufficiently gradual that an acceptably small amount of light is lost along the transitionxe2x80x94the so-called criterion for adiabaticity.
It may be that at least some of the holes have expanded in the heat-treated region.
It may be that at least some, or all, of the holes have collapsed at least partially, or completely, in the heat-treated region.
The pattern of hole collapse and/or expansions may be not circularly symmetric. The birefringence of the fiber may be altered by the lack of circular symmetry.
The fiber may have been narrowed in the heat-treated region.
The photonic crystal fiber may be included in an optical device.
The photonic crystal fiber may be included in a mode-field transformer, the transformer being arranged so that a guided mode propagating through the transformer will have its field distribution changed by propagation through the heat-treated region of the photonic crystal fiber. The shape and size of the field distribution of the guided mode in a PCF depends on the relative sizes of the air holes and their separations from each other. Thus a PCF that has been heat treated to change the sizes of the holes (or narrow down the entire fiber) can act as a mode field transformer.
The photonic crystal fiber may be included in a multi mode to single mode transformer or mode filter, in which the untreated regions of the fiber are multi mode and the heat-treated region is single mode for at least one wavelength of light. Light propagating through the treated region will be forced into a single mode and will remain substantially single mode when it passes into the untreated region, which is capable of supporting other modes; those other modes will, in an ideal fiber, remain unexcited.
The photonic crystal fiber may be included in a fiber input- or output-coupler, in which the photonic crystal fiber is cleaved in the heat-treated region. Such devices can be used to enhance the coupling of light into or out of the end of the fiber if the fiber is cleaved in the region of the heat treatment.
The fiber input- or output-coupler may be arranged so that, when a mode is propagating through the coupler, the mode pattern at the cleaved face will substantially match the mode shape of an external optical element. The external optical element may be a diode laser. Making the mode elliptical or rectangular will allow more efficient launching of light into the fiber from a diode laser source. Indeed, simply expanding the mode size will make easier launching of light from other sources into the fiber.
The fiber coupler may be included in a fiber splice, in which the fiber coupler is joined to one or more other fibers. The joining may, for example, be effected by fusion, by an adhesive or by abutment.
At least one of the one or more other fibers included in the fiber splice may be selected from the following: a cleaved photonic crystal fiber; a fiber coupler, as described above; a standard fiber; or a standard fiber that has been tapered by stretching during heat treatment and then cleaved. Controlled hole collapse provides a way of fusion splicing pairs of PCFs together. First the troublesome air holes are eliminated by their complete collapse in a portion of each fiber, as described above. It is not necessary to stretch the fibers. The fibers are cleaved at the portions where the holes have collapsed, and can then be fusion spliced, since there are no holes to explode and the mode fields will match. (If the fibers are not identical, one or both of them can be stretched so that their outer diameters match. Their mode fields will then be identical.) As an alternative to fusion splicing, two PCFs that have matched mode fields can be mechanically spliced using an adhesive in the conventional way.
A standard fiber tapered to the point where the light is guided by its outer boundary has a similar mode field to a PCF whose holes have collapsed completely and which has also been stretched to the same final diameter. The splice will then be low in loss.
A photonic crystal fiber according to the invention may be arranged so that interaction of light, propagating in the photonic crystal fiber, with the external environment is enhanced or suppressed in the heat-treated region. Such a photonic crystal fiber may be included in an optical device, such as, for example, a mode-field transformer. The interaction may facilitate measurement of a measurand of the fiber""s external environment. The interaction may, for example, be with an external optical element. The optical element may comprise one or more other optical fibers. At least one of the one or more other optical fibers may be a photonic crystal fiber according to the invention or a standard fiber. The spreading out of the mode field caused by collapse of air holes will increase the intensity of the field at the outer boundary of the fiber. Light at the boundary is free to interact with the environment around the fiber. Interactions with the external environment can therefore be enhanced (or suppressed) by hole collapse (or expansion). If the light can interact with a particular property in the environment (for example, the external refractive index), the treated fiber will act as an environmental sensor. If a suitable optical element is placed next to the fiber, optical devices can be built based on the interaction with that element. In particular, the optical element can be at least one other fiber, which may be a PCF or a standard fiber and may have been fused to the first fiber during the heat treatment, in a similar process to the fabrication of fused directional couplers in standard fibers.
At least two fibers in the optical device may have been at least partly fused together by heat treatment.
Similarly, variation of the mode field can be used to control the interaction of the light in the core with any structure that has been introduced into the rest of the fiber. Examples of such structures are diffraction gratings, doped regions (which may be optically pumped to provide gain), or indeed additional guiding cores. Devices that can be based on such interactions include directional couplers, spectral filters, sensors and lasers or optical amplifiers. A photonic crystal fiber according to the invention may be arranged so that interaction of light, propagating in the photonic crystal fiber, with a structure (deliberately) introduced elsewhere in the fiber is enhanced or suppressed in the heat-treated region. Such a photonic crystal fiber may be included in an optical device. The introduced structure may, for example, be one of the following: at least one other core region in the photonic crystal fiber; a grating; or a region of doped material.
The optical device may be included in another optical device such as a directional coupler, a spectral filter, an optical sensor, a laser or an optical amplifier (which may in turn include a directional coupler).
Also according to the invention there is provided a method of producing a photonic crystal fiber, including the following steps: fabricating a photonic crystal fiber having a plurality of longitudinal holes; and heat-treating a region of the fiber so that at least some of the holes in that region have a different cross-sectional area from their cross-sectional area in regions of the fiber which have not been heat-treated, the optical properties of the fiber in the heat-treated region being altered by changing the cross-sectional area of holes in that region.
At least some of the holes may be pressurized during the heat treatment. All of the holes may be pressurized during the heat treatment.
The heat treatment may cause at least some, or all, of the air holes within the fiber to collapse completely in the treated region.
It may be that the heat treatment is not applied in a circularly symmetric manner, so that a pattern of hole collapse and/or expansion that is not circularly symmetric results. The birefringence of the fiber may thereby be altered in the heat-treated region. The polarisation properties of a PCF depend on the structure in the vicinity of the core. Controlled variation of air hole sizes and overall fiber diameter can therefore be used to modify the birefringence of the fiber.
The fiber may be narrowed locally by stretching during the heat treatment.
The heat treatment may change the physical and/or chemical state of at least some of the material within the fiber.